Pre- and post-immunization sera from children receiving OPV with or without simultaneous oral iodine supplementation were coded and tested for neutralizing antibody titers to each of the three types of poliovirus. The study convincingly demonstrated that simultaneous administration of oral iodine did not interfere with the immune response of infants to OPV and that the opportunity of childhood immunization visits can be used to prevent dietary hypothyroidism by oral administration of iodized oil.